Bug 407150

SUMMARY
When I receive mail which is in an older thread, sorting is applied only if I switch to another folder and back again. Individual (unthreaded) mails are sorted correctly.

I usually have kmail running in the background, minimized to the system tray.
Threading is enabled, and I have sorting set to 'by date / time of most recent in subtree'. 

STEPS TO REPRODUCE
1. Select a folder in the kmail window. Enable threading and sorting threads by 'most recent in subtree'. Minimize kmail to the system tray.
2. Wait until you receive an email (or send one to yourself from some other client) that is both
   A. in the aforementioned folder
   B. in a thread that is not currently on top in the message list (i.e. you received some other emails after that thread).
3. Open the kmail window.
4. Observe that the new email is shown, but its thread is not moved to the top. It is still at its previous position.
5. Navigate to another mail folder, then back to the previous one.
6. Observe that the threads are now sorted correctly.

OBSERVED RESULT
Threads in the 'active' folder are sorted only after we navigate to another folder and back again.

EXPECTED RESULT
One of the below behaviours:
A. Threads are re-sorted whenever a new mail check is done (this may be too intrusive)
B. Threads are re-sorted whenever there is new mail AND kmail is minimized.

I think B would be better, as it would prevent threads jumping around in the message list while one is viewing it.

SOFTWARE/OS VERSIONS
Linux distribution: Arch Linux
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.57.0
Qt Version: 5.12.3
Akonadi version: 0.99